1. Field of the Invention
This invention relates to a gate-controlled bidirectional semiconductor switching device such as a triac and, more particularly, to the improvement of the gate sensitivity thereof.
2. Description of the Related Art
A conventional triac is a type of bidirectional semiconductor switching device and is formed to have a cross section such as is shown in FIG. 1. In FIG. 1, numeral 30 denotes an n-type layer, 31 and 32 p-type layers, and 33, 34, 35 and 36 n-type layers. An electrode T1 is formed in contact with the p-type layer 31 and the n-type layer 33, a gate electrode G is formed in continuous contact with the n-type layer 34 and the p-type layer 31, an electrode T2 is formed in continuous contact with the p-type layer 32 and the n-type layers 35 and 36.
The gate electrode G and a portion of the p-type layer 31 situated under the gate electrode G make up the gate structure of a thyristor. A first and a second npn transistor together make up a remote gate structure, the first npn transistor being made up of the n-type layer 33, p-type layer 31, and the n-type layer 30, and the second npn transistor being made up of the n-type layer 34, p-type layer 31, and n-type layer 30. In addition, the n-type layer 34 and the p-type layer 31 make up a junction gate structure.
Four modes, I, II, III, and IV, are provided for turning on the triac having the construction described above. In mode I, the gate structure of the thyristor is used to turn on the triac; that is, the triac is turned on by applying a positive trigger to the gate electrode G when the electrodes T1 and T2 are respectively set at positive and negative potentials. In mode II, the junction gate structure is used, and the triac is turned on by applying a negative trigger to the gate electrode G when the electrodes T1 and T2 are set respectively at positive and negative potentials. In mode III, the remote gate structure is used, and the triac is turned on by applying a negative trigger to the gate electrode G when the electrodes T1 and T2 are set respectively at negative and positive potentials, and lastly, in mode IV, the remote gate structure is used, and the triac is turned on by applying a positive trigger to the gate electrode G when the electrodes T1 and T2 are set respectively at negative and positive potentials.
In order to increase the gate sensitivity of the conventional triac shown in FIG. 1, it is necessary to reduce an invalid current component, this being a current which flows along the surface of a p-type base formed of the p-type layer 31 and does not contribute to an injection current. For this purpose, attempts have been made to increase the resistance of a surface layer of the p-type layer 31 by lowering the impurity concentration thereof, or to interrupt the current flow in the p-type layer 31 by forming an n-type barrier layer therein. In either case, however, it has been found that the gate sensitivity cannot be increased without degrading other main characteristics. For example, any increase in the gate sensitivity is accompanied by a deterioration of the high temperature characteristics, a reduction in the critical rate of rise of the off-state voltage, or dv/dt, at the time of commutation (referred to as a dv/dt value), and the like. Further, from the standpoint of the operational principle of the triac, an emitter formed of the n-type layer 33 must be in the shorted structure, thereby limiting the degree of sensitivity which can be attained by fine control of the impurity diffusion.
Thus, for the above reasons, it has hitherto been difficult to improve the gate sensitivity of the conventional triac to such a degree that the triac can be directly driven by an output signal of a semiconductor integrated circuit (IC), and as described above, in the case of the conventional gate-controlled bidirectional semiconductor switching device, it has proved difficult to increase the gate sensitivity without lowering such characteristics as the dv/dt value.